Method and System for Denser Ultrasonic Transducer Arrays Using Limited Set of Electrical Contacts

ABSTRACT

An ultrasonic transducer and a method for a transmitting an acoustic wave using an ultrasonic transducer comprising a membrane; two or more patterned top electrodes; a pMUT array, wherein the patterned top electrode is arranged as row pin selector and column selector in an N×N array; the pMUT array having N+N electrical contacts; a single unpatterned bottom electrode; a row and column where the electrode is at equal or opposite polarities; and a AC driving voltage is applied to top electrodes with a phase difference of zero or is applied to one electrode to transmit the ultrasonic wave.

BACKGROUND OF THE INVENTION

Micromachined ultrasonic transducers (MUTs) continue to develop as technological elements for machine-human interface and healthcare applications. Recent advances in micro-fabrication technology and techniques have resulted in wider and innovative applications for pMUTs as limitations such as critical dimensions have been resolved to an appreciable stage. Unlike capacitive micromachined ultrasonic transducers (cMUTs), pMUTs do not require high DC polarization voltages and small capacitive gaps, which reduces complexity of driving circuitry and fabrication. In the present invention, a method of electrically addressing elements of a piezoelectric micromachined ultrasonic transducer (pMUT) in a two dimensional array with minimal number of electrical contacts was described by addressing a row and column selection line for each element using only top electrode patterning.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1: Illustration of a top view schematic of array of ultrasonic transducers.

FIG. 2: Illustration of cross sectional schematic of a typical pMUT.

FIG. 3: Illustration of the cross sectional schematic of the proposed pMUT.

FIG. 4: Illustration of an array of connected drive lines with rows and column sections for pMUT electrodes.

FIG. 5: Interferogram image of a piezoelectric micromachined ultrasonic transducer pMUT demonstrating a fundamental mode resonance.

FIG. 6: Illustration of different top electrode metallization schemes for dual electrode configuration for row/column addressing of 2D PMUT arrays.

FIG. 7: Illustration of a top electrode routing for top electrode only row/column addressing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Typically pMUT arrays have few elements (less than 100 vibrating membranes) and as such, connecting each membrane to the corresponding driving circuitry is achieved with wirebonding, flip-chip bonding or any other one-to-one correspondence of each membrane electrode pair to driving electronics. For integration of a large number of membranes for advanced, high resolution ultrasonic imaging, the number of required connection can be too complex and cumbersome for direct implementation. In those cases, a row/column addressing of each acoustic pixel is more feasible and less complex. For the definition of the piezoelectric actuated membranes, it may be convenient not to pattern the bottom electrode of the pMUT, as it may interfere with the crystalline quality of the thin film, which in turn affects the piezoelectric coefficients of the piezoelectric layer. This imposes an extra difficulty on implementing the row/column addressing in a dense pMUT array.

An embodiment is a method for designing an ultrasonic transducer (UT) array that can be dynamically addressed by two or more top electrodes, organized as row pin selector and column selector, turning on/off any element that is selected by the row/column selector electronic circuitry. The bottom electrode is a single electrode that is not patterned. This method of addressing individual pMUT elements in denser arrays provides for the use of limited electrical contacts. A driving electronic circuit including a row and column selector, at least on electronic circuit oscillator, which may be tunable or not, and amplifiers for signal conditioning may be connected to the UT array, either from an on-chip electronic circuitry, a different electronic chip connected to the UT array chip or from a chip or discrete electronic components mounted on an external board. In a further embodiment, the fabrication of a complex 2D beamforming ultrasonic array is enabled to increase the number of independently actuated transducer count from 4 to thousands of elements.

In yet another embodiment of this invention relates to a design of top metallization layer (or layers) in order to route the electrical connections for row/column addressing, where microelectro-mechanical systems (MEMS) based piezo machined ultrasonic transducers (PMUT) are arranged in complex arrays of column and row based electrical signal driving. An ultrasonic transducer capable of generating and detecting acoustic waves, consisting of a piezoelectric deformable layer between two or more electrodes and surrounded by structural thin-film or films forming a suspended membrane. The top electrodes are used to transmit the ultrasonic wave using an electrical signal. The individual pMUT of an array is addressed through phase shifted voltage signals. The membrane can be either clamped or possessing holes, where the shape is circular, rectangular, square, or any other two-dimensional shape.

EXAMPLE 1 Design of a Novel N×N pMUT Array

In reference to the drawings, the reference numerals designate identical or corresponding parts throughout the several views. FIG. 1 illustrates an array of piezoelectric micromachined ultrasonic transducers on which a mechanism of actuation is based on the piezoelectric effect, where an electric field applied along a selected crystal orientation of a piezoelectric crystal leads to the mechanical deformation of such crystal. The electric field is created by means of a pair of electrodes driven by an external electric circuit by means of a voltage difference on the electrode pair. 100 illustrates a single pMUT among a number of other pMUT's positioned on a circuit board 102.

FIG. 2 demonstrates a cross-sectional schematic of a typical pMUT. In the cross sectional view, component 200 illustrates a top electrode situated at the top of the pMUT, it is placed directly on top of a piezoelectric layer 202 which separates the top and bottom electrode 204. Underneath the bottom electrode is a structure layer 206, the thickness of this layer allows the adjustment of the tuning frequency of operation of the pMUT. Beneath it is an isolation dielectric 208 and a support structure made of silicon 210.

The piezoelectric layer is deposited onto a membrane which is fabricated using techniques of microfabrication known in the art. As such a suspended membrane can mechanically deform and oscillate if a time-varying electrical signal is applied to the pMUT electrodes. Reciprocally, if an oscillating mechanical force, such as an acoustic pressure, is applied to the membrane, this will vibrate and in turn a time-varying electrical signal will be detected at the electrodes. In order to obtain high crystalline quality, with low defect density piezoelectric material it is important to maintain a continuous bottom electrode made of a metal with the suitable microscopic properties for the growth of crystalline layer with matching lattice parameters.

In the present embodiment, pMUT devices were patterned on the top electrode while the bottom electrode was not patterned in the pMUT devices. Specifically, as bottom electrode patterning is detrimental to the device performance, affecting the quality of the piezoelectric layer. As such the bottom electrode covers all the bottom surface of the microchip where the pMUT array is manufactured and was be electrically grounded for reduction of the cross-talk between the different elements of the array. At least two top electrodes are defined on the top of the piezoelectric layer of the pMUT with at least one being driving by the row electrical signal and at least one driven by the column electrical signal. The electrodes are made of electrical conductive materials, typically but not limited to metals. Examples of such metals are molybdenum, aluminum, nickel, platinum, titanium, cobalt, tungsten, and similar metals.

By using electric bipolar signals, the row and column top electrodes are set at opposite polarities (180° phase shift of equal level signals) turning off the vibration of the membrane. If both signals have the same polarity and level (0° phase shift) the piezoelectric induced vibration will be induced on that element. Further, by using the opposite polarity mode and a driving frequency corresponding the resonance of the higher mode of the membrane that matches the vibration pattern of the out-of-phase dual electrode configuration, the operation frequency of the pMUT was changeable.

In the three electrode configuration, a fundamental mode (also known as first mode) is obtained for the following conditions. First, the driving voltage/current source is sinusoidal with a frequency corresponding to the first mode of the membrane, and second, the AC driving voltage is applied to both top electrodes with a phase difference of 0 (zero) degrees or the AC driving signal is applied to only one of the electrodes, for which condition, the dynamic displacement will be half of that obtained from driving the two top electrodes in phase. If the two top electrodes are out of phase by 180 degrees, the vibration is heavy damped, with no displacement measurable, therefore shutting down the mechanical oscillation even with a voltage signal applied to both top electrodes. Advanced beamforming patterns are formed by two dimensional addressing of dense (more than 256×256 element) pMUT arrays.

In FIG. 3, a cross-sectional schematic of the pMUT embodiment is shown. The top layer is a piezo-micro-electromechanical systems (MEMS) and the bottom layer is a complementary metal-oxide semiconductor (CMOS) based driver and signal processing application specific integrated circuit (ASIC). A through-silicon via (TSV) layer electrically isolates the CMOS layer and the MEMS layer, with TSV being the only channel of vertical electrical connection through the silicon wafer or die. The stand-offs isolate the vibrating diaphragm from the TSV layer of the CMOS wafer. Thickness of stand-off may be 1 to 3 um. Thickness of the TSV layer would be 100 to 250 um. The aspect ratio of vias would be 20 to 30. Thickness of MEMS diaphragm may be 1 to 4 um.

The CMOS layer is represented as the bottom layer in FIG. 3. A 180 nm Bipolar-CMOS-DMOS (BCD) technology was used to fabricate the ASIC, which can be used to drive the MEM device, and to process the electronics signals from the MEM layer. The driving circuit may contain a high voltage pulser that can boost 3.3 to 5 Volts (from CMOS input) to 10-60 Volts. The signal processing ASIC involves time gain compensation and low noise amplifier to provide a gain of 20 to 40 dB. The CMOS layer is represented as the bottom layer in the illustration.

FIG. 4 illustrates complex arrays formed by connecting the different drive lines for row and column selection, using two different top electrodes (one for row selection and another for column selection), besides the bottom electrode reference (for example, ground). The electronic pulse controller changed the pulse polarity from, for example, 4V (represented by 1) to −4V (represented by I). When both top electrodes are pulsed with the same polarity, the element is vibrating at the resonant frequency (selected as carrier frequency of the pulse modulated signal). The membrane vibration is off when the two electrodes are driven by pulses with opposite polarity also significant reduction of contacts pads.

For an N×N array, instead of N2 contacts, only 2N are needed. Another benefit of this design is a high contrast on-off ratio without floating grounds/signals inducing residual piezoelectric to mechanical transduction.

EXAMPLE 2 Exemplary Fabrication of N×N pMUT

In one embodiment, a three electrode configuration of the fabricated pMUT is shown in FIG. 5. The top electrodes are driven in a push-pull configuration by opposing polarity electrical signals. The single bottom electrode was not patternsed and is not shown.

For fabrication, aluminum nitride (AIN) was used as the active piezoelectric layer with a thickness of 111 m, positioned between two molybdenum electrodes (0.2 11 m thick each) and deposited over a 6 11 m silicon passive membrane layer. The 200 11 m radius membrane was released by backside DRIE etch. The top electrode diameter was 65% of the total device diameter that further divided in two halves, with each connected to a different electrical connecting pad. The bottom electrode was not patterned and covers fully the bottom side of the membrane.

The interferogram depicted in FIG. 5, showed one state of the behavior of the pMUT. The top electrodes are driven an in-phase 4 Vpp AC signal scanning from 1.975 MHz to 1.997 MHz, with the fundamental mode resonance at 1.99 MHz. When switching one of the electrodes to inverted signal polarity (1800 phase shift) no displacement is observable (shown in FIG. 5).

The configuration was developed into the different models represented in FIG. 6, as represented in the four examples shown for row/column addressing of each PMUT membrane using only top electrode patterning.

For more complex arrays, FIG. 7 shows an EV2 test design of top electrode routing for top electrode only row/column addressing. The illustrated design has a more complex electrical routing that were simplified into denser arrays. The design of the N×N array may be arranged in thousands of electrode elements.

Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 

1. An ultrasonic transducer that transmits an acoustic wave comprising: a. a membrane; b. two or more patterned top electrodes; c. a pMUT array, wherein the patterned top electrode is arranged as row pin selector and column selector in an N×N array; d. the pMUT array having N+N electrical contacts; e. a single unpatterned bottom electrode; f. a row and column where the electrode is at equal or opposite polarities; and g. a AC driving voltage is applied to top electrodes with a phase difference of zero or is applied to one electrode to transmit the ultrasonic wave.
 2. The ultrasonic transducer of claim 1, wherein turning on or off any electrode using a multiplex switch that is selected by the row or column selector address.
 3. The ultrasonic transducer of claim 1, wherein arranging 2×2 or more independently actuated transducers forms a 2D beamforming ultrasonic array.
 4. A method for a transmitting an acoustic wave using an ultrasonic transducer, comprising: a. arranging a pMUT array having two or more electrodes as a row pin selector and column selector in a N×N array on a membrane; b. displaying the pMUT array having N+N electrical contacts; c. aligning a single unpatterned bottom electrode under the member; d. setting a row and column where the electrode is at equal or opposite polarities; e. applying an AC driving voltage to top electrodes with a phase difference of zero or is applied to one electrode; and f. transmitting the ultrasonic wave.
 5. The ultrasonic transducer of claim 4, wherein a multiplex switch is used for turning on or off any electrode that is selected by the row or column selector address.
 6. The ultrasonic transducer of claim 4 forms 2D beamforming ultrasonic array, wherein the 2D beamforming ultrasonic array ranging from 2×2 or more independently actuated transducers. 